1. Field of the Invention
The present invention relates generally to light weight bearings and methods of manufacturing same and, more particularly, to bearings of the type including a light weight outer sleeve or carrier which is interference fit connected with a heavier internal insert, liner or load bearing surface. Assembly of bearings of this type includes developing a relative temperature differential between the outer sleeve and the inner liner whereby, after inserting the liner into the sleeve, a radial interference connection is established when the sleeve and inner liner composite connection reaches an equilibrium temperature.
2. Description of the Prior Art
Prior to the instant invention, several alternative approaches have been explored in the construction of light weight bearings with varying degrees of success. The basic engineering problem to be solved has been one of providing suitable bearing working surfaces, such as of a leaded bronze material, to absorb the bearing loads from rotating parts while simultaneously employing lighter weight materials, such as aluminum alloys, to transmit the mechanical loads and the frictional heat generated by the working surfaces, to a larger overall housing structure.
A number of adhesive bonding methods have been attempted whereby a leaded bronze insert is joined to an outer light weight sleeve carrier using an adhesive substance. According to this design the leaded bronze insert provides a bearing working surface while the light weight carrier provides mechanical integrity. However, bearings using the adhesive bonding methods often experience fluid leakage between the bonded pieces. Of course, this fluid leakage is not usually designed into the product and typically degrades bearing performance. In addition, bonding methods are sensitive to bearing geometry specifications as well as process variations, cleanliness, and bonding material composition. The variations in assembly processes include surface preparation and cleanliness, adhesive application and its uniformity, and curing methodologies. In-service conditions such as loading, pressure, temperature and fluid properties can mechanically damage and/or chemically alter the bond joint causing it to fail which, in turn, degrades the service life of the apparatus and may induce secondary failures.
In addition to the acute sensitivity of these bearings to the bonding process variations, the adhesive used to connect the components often tends to act as a thermal insulator, thus inhibiting the ability of the leaded bronze insert to dissipate the heat developed through friction, into the surrounding structure. Overall, bearing load capacity is impaired due to heat build-up, which directly reduces the viscosity and hydrodynamic load capacity of the working fluid between the moving parts in the bearing. Further, adhesives are subject to possible long term degradation in service due to thermal exposure such as extremes in temperature, thermal cycling and differential part expansion. Still further, the adhesives are susceptible to chemical attack which can lead to catastrophic failure of the bond joint itself. Lastly, process variations make it virtually impossible to accurately predict bond joint life. One solution is to calculate an anticipated bond joint life and replace the bearing well in advance of its life expectancy. However, this is usually commercially unfeasible.
An alternative strategy to the various adhesive bonding methods mentioned above includes the use of elastomeric seals between two or more light weight bearing materials which are mechanically assembled into a composite bearing structure. Such bearings, however, are susceptible to leakage due to degradation of the seals such as by chemical attack, thermal cycling, differential expansion of the various bearing parts and wide temperature extremes. In addition, elastomeric seals tend to wear prematurely due to relative motion between or among the various pieces comprising the composite bearing structure. Weakened bearing seals may extrude through clearance gaps or otherwise rip, tear, or abrade. Further, overall bearing load capability may be impaired due to the effects of manufacturing tolerances of each of the individual parts comprising the composite bearing assembly. This can result in excessive misalignment between the working surfaces of the bearing and the associated rotating or otherwise moving parts. Additionally, discontinuities such as clearance gaps between the parts disrupt the efficient conduction of frictional thermal energy away to the surrounding structure, impairing the overall load carrying ability of the bearing. Lastly, as the number of pieces comprising the composite bearing structure increases, the likelihood of improper assembly increases as does the overall labor, raw material, and resultant manufacturing cost. This becomes significant over the total product service life when multiple assemblies or overhauls occur.
A third alternative light weight bearing construction strategy includes a metallurgical bonding approach whereby a bronze liner is brazed or soldered to a light weight aluminum carrier. Casting processes have also been attempted with varying degrees of success. Overall, metallurgical bonding overcomes most of the leakage, heat conductance and part size tolerance problems associated with the adhesive bonding and multiple piece elastomeric sealing methods discussed above. However, the differential thermal expansion experienced between the bronze liner and aluminum carrier tends to negatively influence overall durability problems, particularly in bearings of large diameter or high length/diameter (L/D) ratios. Also, material properties such as strength, ductility, and thermal expansion can very within a single bond zone, particularly in cast bearings, due to alloying, different rates of cooling or solidification and for other reasons.
Properties of the metallurgical bonds also tend to vary over time, due to thermal exposure, diffusion and migration of constituents, and other effects such as work hardening or fatigue. Metal matrix composites (MMCs) have been used in an attempt to reduce or eliminate differential thermal expansion in bearings which rely on metallurgical bonds. However, problems have been encountered with reduced material ductility in the MMCs, and material property variability remains a troublesome engineering problem. In any case, light weight bearings employing the metallurgical bonding techniques also remain highly susceptible to process variations and tend to be quite expensive to produce.